Do you also keep your cash under your mattress, instead of into a bank account?
Coinbase != bank.
The banking system has a safety net for recovery of stolen funds. Coinbase has pretty much jack squat in that sense.
True, Coinbase is insured against loss from its hot wallet. But if Coinbase were to suffer a loss from its cold wallet, you'd be left with zilch.
Likewise, Coinbase's terms of service put all of the responsibility for security on you, the user.
Your bank is a different story. It's FDIC insured. It has in place numerous security measure that enable it to claw back any digital theft and make you whole.
The author has to eat the loss, as per the user agreement.